Conventionally, a semiconductor substrate in a semiconductor device and a mounting substrate have been electrically connected by wire bonding.
However, with electrical connection by the wire bonding, it is necessary to provide an area in the vicinity of edges of the semiconductor substrate and the mounting substrate to which an end of a wire that is used for the electrical connection is connected. Therefore, there has been a problem with the electrical connection by wire bonding that a mounting structure of a semiconductor device becomes large in size.
Moreover, the electrical connection by the wire bonding has a long connection distance between the semiconductor substrate and the mounting substrate. This increases inductance of the wire, and thereby makes it difficult to carry out high speed driving.
In order to solve these problems occurring with the wire bonding, flip chip mounting has been proposed in the recent years.
The flip chip mounting is a mounting method in which (i) a bump to be used for bonding with a mounting substrate is provided on a functional surface of the semiconductor substrate, (ii) the functional surface of the semiconductor substrate and a surface of the mounting substrate are positioned so as to face each other, and (iii) the bump and an electrode on the surface of the mounting substrate are bonded together.
There are several ways of flip chip mounting; among the several ways, mounting methods by means of metal bonding allow attainment of high mounting reliability, and can also have a fine pitch. For such reason, the mounting methods by means of the metal bonding have become the mainstream of the flip chip mounting.
Furthermore, among the mounting methods by means of the metal bonding, a mounting method by means of metal bonding using Cu and solder can be realized at low costs, and can attain a high mounting reliability to a substantially same degree as that of a mounting method by means of metal bonding using Au and solder.
In the flip chip mounting method in which metal bonding is carried out, heat is applied to the metals used for carrying out the metal bonding so as to bond these metals together. Heat generated by this application of heat causes interdiffusion of metals.
However, if an amount of a metal used for bonding is great in carrying out the metal bonding, the metal used for the bonding cannot be completely formed into an intermetallic compound. As a result, a bonding part which has been subjected to the metal bonding includes the intermetallic compound and the metal used for the bonding. Bonding strength is extremely weak at an interface between the intermetallic compound and the metal used for the bonding. Hence, application of a just minute force causes occurrence of a breakage.
One example of this is as described below. With the mounting method for flip chip mounting by means of metal bonding of Cu and solder, the Cu and solder are heated, so as to bond the Cu and the solder together. The heat generated due to the application of heat to the Cu and solder causes the interdiffusion of the Cu and the Sn (tin) contained in the solder. However, if the amount of solder is great, the solder cannot be completely formed into the intermetallic compound. This causes a bonded part of the Cu and the solder to include the solder, as well as a Cu3Sn layer and a Cu6Sn5 layer which are the intermetallic compounds. The interface between the Cu6Sn5 layer and the solder is extremely weak in bonding strength. Therefore, a breakage occurs just by application of a minute force to the interface.
Patent Literature 1 (Japanese Patent Application Publication, Tokukai, No. 2002-110726 A (Publication Date: Apr. 12, 2002)) discloses a technique which applies a thin layer of bonding material, namely, for reducing an amount of the bonding material used, with a semiconductor device in which the bonding material used for bonding is stacked on at least one of electrode material of a semiconductor substrate and electrode material of a mounting substrate. If the amount of the bonding material is reduced, the bonding material can be completely formed into the intermetallic compound; thus, it would be possible to suppress the occurrence of the problem related to the breakage.
However, as in the technique disclosed in Patent Literature 1, if the amount of the bonding material is decreased in amount, the bonding part itself becomes narrow, which causes the mounting reliability to decrease.